When you bring a pot of bones and water up to a boil, the bones will release what are usually called impurities. They’re not particularly impure, they’re really just food particles and fat. If you leave them in the stock, though, they will eventually disperse into the broth and give it a cloudy, grayish color. So to get a clear, pretty broth that looks as good as it tastes, that’s the stuff you want to get rid of. Here are some thoughts about skimmery from Marco Canora:
— When you’re adding water to the pot, make sure it’s a couple of inches over the level of the bones. Those couple of inches aren’t part of a
magical ratio; it’s so you can fit your ladle in there. If you don’t leave enough headspace, your ladle won’t fit, and you won’t be able to skim properly.
— When it’s time to skim, slowly lower the ladle into the pot. As soon as the edge of it breaks the surface tension on the top of the water level, freeze. Don’t move the ladle all around the top of the pot; you’re going to scoop up too much of the good stuff. The scum should flow right in.
— It’s easy to underestimate the amount of time it will take to bring the bones and water to a boil. The scum will continue to rise to the top in the first hour or two; repeat the above process as needed until there’s no more scum to skim.